Nanopb - protocol buffers with small code size

Nanopb is a plain-C implementation of Google's Protocol Buffers
data format. It is targeted at 32 bit microcontrollers, but is also fit for other embedded systems with tight
(2-10 kB ROM, <1 kB RAM) memory constraints.

License

This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable
for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it
and redistribute it freely, subject to the following restrictions:

The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If
you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.

Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.

This notice may not be removed or altered from any source distribution.

Similar projects

As of 2015, protobuf-c is the most viable
alternative, should you require something more than what nanopb can provide. It has more complete
support of Protocol Buffers features (such as easy to use unbounded recursive messages), but relies
on dynamic memory allocation and the code size is quite large (more than 20kB binary).

There are also several other projects that aim to create a Protocol Buffers implementation
suitable for embedded use. As of 2015, they all seem to be unmaintained:
lwpbProtobuf-Embedded-Cempb.